Process of continuous casting of steel

ABSTRACT

During the starting phase of continuous casting, a fast melting, slag producing powder is added and replenished subsequently by a high melting powder.

United States Patent Schrewe et al.

[54] PROCESS OF CONTINUOUS CASTING OF STEEL Inventors: lhns Schrewe, Duiaburg-Ungelaheim; Gerri Diederich, Krefeld; PM Wnlih,

Buederich; Emil Planning, Duisburg- Buchholz, all of Germany Assignee: Mnlmeammn Aktiengeaelllehafl, Duesseldorf, Gennany Filed: Mar. 23, 1970 Appl. No.: 22,067

Foreign Application Priority Data Mar. 21, 1969 19 15 303.8

US. Cl. ..l64/55, 164/82, 164/281 Int. Cl 27/18 FleldofSearch ..164/53, 55,56, 73, 82,281,

[56] References Cited UNITED STATES PATENTS 3,411,566 ll/l968 Astrov etal ..l64/S$ X 3 52,936 9/1962 3,318,363 5/1967 3,519,059 7/1970 Voskoboinikov et al. 164/53 Primary Examiner-J. Spencer Overholaer Assistant Examiner-John E. Roethel Attorney-Smyth, Roston & Pavitt [57] ABSTRACT During the starting phase of continuous casting, a fast melting. slag producing powder is added and replenished subsequently by a high melting powder.

PROCESS OF CONTINUOUS CASTING OF STEEL The present invention relates to a method and process for applying and feeding casting powder into a mold cavity during continuous casting of steel, particularly after a particular filling state and level of molten steel has been established in the pouring cavity of the mold.

Continuous casting of steel is carried out today almost exclusively under utilization of a casting powder for establishing a slag layer between the skin of the casting ingot and the wall of the mold. These powders are characterized by relatively low melting points, particularly below the melting point of the steel used for casting. As the powder is applied to the surface level in the mold it melts and a rather thin, liquidous slag is formed and flows in between the solidifying casting skin or shell and the wall of the mold cavity. Casting powders are known which form a more or less thin slag, others form a more viscous slag. These different kinds of casting powders are used in dependence upon the quality of the steel, whereby the same powder is used throughout the entire casting process, from beginning to end. It is, furthermore, well known to charge the mold initially, i.e., at the beginning of the casting process, with molten material of the same consistency as it is to be used as powder subsequently during the casting process.

These methods, however, have certain disadvantages. Casting powder having a relatively high melting point requires a period of at least several minutes for melting, after having been deposited in the mold. Even the melting of casting powder having relatively low melting point still takes too long during the starting phase, particularly as the powder is required to sustain slag formation subsequently, under regular, relative long'term manufacturing conditions for producing casting ingots of satisfactory quality. It has been found that during the period required for melting of either type of casting powder cracks may fonn in the skin particularly of rectangular slab ingots having width in excess of 800 millimeters. Even the known advance melting of a casting powder having relatively low melting point is met with considerable difficulties during operation and production. First of all, melting of a quantity of powder in advance is rather time consuming and was found difficult to accomplish because as the material melts the part that is already molten forms a layer which constitutes a kind of heat shield, screening to some extent the remaining, still powderous portion from further influx of heat, thus, retarding melting thereof. After a rather long period of time all of the powder may finally be molten, but the melt may react with the fireproof material of the melting ladle in such a manner that the desired consistency of the slag cannot be maintained any longer.

it is, therefore, the task of the present invention to solve the problems arising from the aforementioned dimculties and to suggest a method and process having as its ultimate purpose the production of a crack-free casting string or ingot. ln accordance with the present invention it is suggested that during the casting process at least two different casting powders are employed. These casting powders differ as to their rate of melting and as to their behavior during the melting process generally. The powders are fed sequentially to the level of the melt in the mold or pouring cavity, in that first a casting powder is applied which liquefies rather quickly, while a different, high melting powder is applied subsequently, beginning preferably by the time at least half of the previously applied, first powder has already been used up. The second powder is then used throughout continued casting, subject only to a particular terminating phase of the process.

The second powder differs from the first one, for example, by a higher melting point or greater heat of fusion or both; the relatively quick liquefying casting powder to be used first is, therefore, characterized by a relatively low melting temperature and/or a relatively low heat of fusion as measure of thermal energy required for melting the material. The powder to be used first should be selected so as to obtain sufficient amounts of slag right at the beginning of casting and as fast as possible, even though this particular powder may melt too fast, thereby developing slag at too high a rate, if also used after the transitional period of the starting phase throughout the main phase of casting. On the other hand, selection of the second powder can be optimized as to sing production during that main phase irrespective of poor performance of such a powder ifit were also used in the transitional starting phase.

The resulting effect of producing fut luflicient amounts of slag as obtained by using a quick liquefying casting powder first, can be enhanced by including additives in the powder having exothermic characteristics, such as aluminum, silicon, alkali metal or oxygen carrier such as iron oxide or the like. These additives quicken the liquefaction of the powder in the starting phase. Upon contact with the liquid steel the aforedescribed, fast liquefying powder is heated speedily above its melting point of about 800 Centigrade. Immediately a film of liquid slag is formed between the outer surface of the skin of the casting string or slab ingot and the wall of the mold cavity. Moreover, that slag flows directly over the surface of that skin which has just been established. In dependence upon the width of the slab, for example, within the range from 800 to 2,100 millimeters, one will charge the mold cavity with 5 to It) kilograms of the fast liquefying, slag producing casting powder. These quantities sutfice in order to accompany inception and initial production of a casting string having length of about 1.5 meter, to be produced without rupture.

As this first powder liquefies and flows ofl as slag it is stepwise or continuously replaced by a casting powder of, for example, a higher melting temperature. The melting point of the casting powder to be used now, and to be used continuously thereafter until termination of casting, is about l,l50 Centigrade. It is of advantage to begin already replenishing of the powder content by adding the second, higher melting casting powder when only about half of the initial first casting powder has molten. One can recognize this state and condition in the mold by a change in color of the surface of the powder therein. It was found to be of additional advantage to establish a transition phase in that after the initial charge with fast liquefying powder, a mixture of the two types of powder is used before using the second high melting powder exclusively.

By way of example, it was found that the following powders can be used with advantage to practice the invention. The first, relatively fast liquefying powder consists of (all percentages by weight):

SiO 29% C00 221 mo use up, use "0, 0.55% Zn 0.05: rs 0.3% Cu 0.05% Mn [5% F o, use F ms c 2.7a w o 3.31 too 2.55

The second higher, i.e., slower melting powder may consist of:

500, we 00 at. mo 2% AI,O, mi

T60. ass Pb 0.11 n.0, 0.01 c as r 13* No.0 us up 2.5%

The second powder is added during the continuous casting process at a rate of 0.6 kilograms per metric ton of steel. Shortly before termination of casting it may be of advantage to apply a powder that is burnable and/or remains powderous, for example sawdust, and to be used in lieu of the high melting, liquefying casting powder. This change facilitates termination of the casting process as well as solidification of the end portion of the string by means of water. Upon utilizing high melting casting powder up to termination of the casting process. not all of the added casting powder may have been used up exactly by the time casting is actually tenninated so that a slag layer remains on the head of the string. As cooling water intended for enhancing solidifying of the end portion of the casting string hits such layer of slag instead, the slag solidifies and prevents further heat transfer from the end of the string. Upon removing the string from the mold, its end may actually melt again when passing through and along the withdrawal rolls, and liquid steel may seep out of the string. Consequently. the string has to be stopped but this, in turn, results in strong local cooling, possibly producing cracks. On the other hand, if the end of the string is covered with still powderous material, the cooling water will flush the powder away, and now the string remains uncovered by slag and is directly exposed to the coolant. Hence. the string end will cool and properly solidify.

The invention is not limited to the embodiments described above, but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be covered by the following claims.

We claim:

l. in a method for continuously casting of steel using a mold cavity, comprising the steps of, first applying to the liquid steel in the mold a slag producing powder having characteristics of comparatively fast liquefication upon contacting the liquid steel in the mold; and

subsequently applying a different, higher melting slag producing powder, to the liquid steel in the mold.

2. Method in accordance with claim I the second step overlapping the first step in that adding the second higher melting slag producing powder to the molten steel is begun when at least about half of the melting powder applied in accordance with said first step is used up.

- 3. Method as set forth in claim 1, the first step preceded by the step of preparing a mixture ofa low melting slag producing powder and exothermic additives to be used in the first step.

4. Method as set forth in claim 1, there being an intermediate step interposed between the first and second step, said intermediate step comprising adding during melting of the powder as applied in accordance with the first step a mixture of that powder and of the powder to be used during the second step.

5. Method as set forth in claim I, and comprising the additional step, towards the end of the casting, of applying a casting powder which remains powderous at the operating temperatures.

6. Method as set forth in claim I, and comprising the additional step, towards the end of the casting, of applying a casting powder which burns at the operating temperature. 

2. Method in accordance with claim 1 the second step overlapping the first step in that adding the second higher melting slag producing powder to the molten steel is begun when at least about half of the melting powder applied in accordance with said first step is used up.
 3. Method as set forth in claim 1, the first step preceded by the step of preparing a mixture of a low melting slag producing powder and exothermic additives to be used in the first step.
 4. Method as set forth in claim 1, there being an intermediate step interposed between the first and second step, said intermediate step comprising adding during melting of the powder as applied in accordance with the first step a mixture of that powder and of the powder to be used during the second step.
 5. Method as set forth in claim 1, and comprising the additional step, towards the end of the casting, of applying a casting powder which remains powderous at the operating temperatures.
 6. Method as set forth in claim 1, and comprising the Additional step, towards the end of the casting, of applying a casting powder which burns at the operating temperature. 